In the aircraft industry various components of an aircraft wing are secured together by fasteners such as rivets and slugs. A slug is a metal fastener which typically does not have a preformed head and which has both ends deformed during riveting. A rivet is a metal fastener which has a preformed head and has only the tail deformed during riveting. Because of the large number of slugs and rivets utilized to produce a single aircraft wing, and also because of the requirements of virtually indefinite life of the deformed fasteners, much attention has been given in the industry to various methods and apparatus for upsetting. One upsetting method and apparatus is shown in U.S. Pat. No. 4,908,928. When practicing the method and apparatus disclosed in the aforementioned patent, upper and lower upsetting rams, (which are typically referred to as riveting rams), are carried by cylinder assemblies, the cylinder assemblies being mounted in upper and lower portions of a generally C-shaped frame, such as that shown schematically in FIG. 1 of U.S. Pat. No. 4,864,713. According to the method and apparatus of U.S. Pat. No. 4,908,928, after the upper and lower riveting rams have been snugged against the slug in the workpieces, fluid is simultaneously introduced into the cylinders behind each of the upper and lower riveting rams to move the rams together at substantially the same rate with respect to the cylinders. As the upper and lower riveting rams are moving at the same rate with respect to the cylinders there is little movement of the workpiece during upset of the slug. In other words, the workpieces will not move, or will barely move, relative to a fixed workplane. Movement of the workpieces with respect to a fixed workplane is known in the industry as "wink". If the workpieces are winked or moved during the squeeze cycle of an upsetting or riveting process, they will have a tendency to oscillate before returning to their original position. This oscillation could delay the next forming operation or any other subsequent operation. Greater uniformity of the bulging of the fastener may be achieved if there is limited movement of the workpieces during upset, which is desirable for fastener fatigue life cycles. The limited movement of the workpiece during slug forming or riveting permits clamping the workpieces into a rigid fixture which guarantees the accuracy of workpiece geometry. Since the fastening tooling does not "wink" or lift the workpieces, there is a reduced chance that the tooling will mar the surface of the workpiece which is engaged by the tooling.
When using the machine of U.S. Pat. No. 4,908,928, many of the foregoing objectives have been achieved. However, the foregoing machine included a midpoint supported C-frame which can be shifted vertically, and which can also be rocked and tilted so that the riveting rams may be positioned perpendicular to the surface of the upper workpiece which is held in a stationary manner. However, in some instances, it is desirable to support the workpieces, which may be an aircraft wing, for vertical and titling movement while supporting the riveting rams for X and Y movements only in the horizontal plane. In this situation, the riveting rams are typically supported by upper and lower portions of a large C-shaped frame, only a lower portion of the C-shaped frame being rigidly supported. When using this form of device, the upper portion of the C-shaped frame may deflect more during upsetting of the fastener than the lower portion of the C-shaped frame. When there is greater deflection of the upper portion of the C-shape frame, it can be appreciated that the workpieces being engaged by the riveting rams will be winked as the center point between the rams will move upwardly as the upper portion of the C-shaped frame is being deflected during upsetting. In some applications this winking is undesirable and therefore it is desirable that a method in apparatus be developed which will compensate for frame deflection.
In a more recent patent application about to be issued Oct. 29, 1991 as U.S. Pat. No. 5,060,362, a method and apparatus are disclosed for overcoming deflection of the upper portion of the C-shaped upsetting apparatus, the lower portion of the apparatus being mounted upon the floor. The design shown in U.S. Pat. No. 5,060,362 utilizes the same concepts as shown in the previously mentioned U.S. patent wherein a single master cylinder assembly in turn drives two slaved cylinder assemblies to provide fluid for both the upper and lower cylinders. Additional, fluid is added into the fluid lines for the upper upsetting cylinder assembly by means of a frame compensation booster cylinder assembly. The fluid flow into the frame compensation booster cylinder assembly is controlled by a servo valve, with feed back being provided by a linear transducer associated directly with the frame compensation booster assembly. While this form of device has proved to be satisfactory in its operation, it is somewhat complex.